THC in breast milk:
Does THC in breast milk of cannabis using mothers harm the baby?

Answers:

Chao and colleagues
Lactating monkeys chronically receiving 2 mg THC per kilogram body weight orally either two or five times weekly were given a small dose of radiolabelled THC mixed with the THC. During the 24-hr observation period, approximately 0.2% of the labelled THC appeared in the milk.
Modified according to: Chao FC, Green DE, Forrest IS, Kaplan JN, Winship-Ball A, Braude M. The passage of 14C-delta-9-tetrahydrocannabinol into the milk of lactating squirrel monkeys. Res Commun Chem Pathol Pharmacol (1976 Oct) 15(2):303-317.

Susan Astley and Ruth Little
The present study investigated the relationship between infant exposure to marijuana via the mother's milk and infant motor and mental development at one year of age. 136 breast-fed infants were assessed at one year of age for motor and mental development. 68 infants were exposed to marijuana via the mother's milk. An additional 68 infants were matched to the marijuana-exposed infants with regard to maternal alcohol and tobacco use during and after pregnancy. Marijuana exposure via the mother's milk during the first month after birth appeared to be associated with a decrease in infant motor development [development of movements] at one year of age. There was no association between marijuana exposure during the third month after birth and motor development. There was no association between marijuana exposure during the first and third month and infant mental development.
This result should be interpreted cautiously due to the preliminary nature of this study. One cannot deduce form the results presented here that marijuana exposure during lactation impairs infant motor development at one year of age. Marijuana exposure during lactation appeared to impair infant motor development, but it does not mean that the relationship is one of cause and effect. Marijuana exposure during pregnancy, passive exposure to marijuana smoke in the air and the quality of interactions between mother and child are three factors that could potentially confound the observed association in this study.
Modified according to: Astley SJ, Little RE. Maternal marijuana use during lactation and infant development at one year. Neurotoxicol Teratol 1990;12(2):161-168.

Franjo Grotenhermen
Small quantities of THC pass into the milk of cannabis using mothers. In a study in monkeys who chronically received THC, 0.2% of the THC appeared in the breast milk. According to that, daily use of 50 mg of THC (e.g. 1 gram marijuana with a THC content of 5%) would result in daily 0.1 mg THC in breast milk.
Chronic cannabis use by the mother leads to an accumulation of THC in the milk. Milk of cannabis using mothers may show significantly higher concentrations of THC than their blood. In one study the concentration of THC in the milk was 8.4 times higher than in blood plasma. Assuming a relatively high medium concentration of 10 ng/ml (nanogram per millilitre) THC in the plasma of a cannabis user and a tenfold higher concentration in the milk (100 ng/ml), this would result in overall 0,07 mg THC in 700 ml milk. 700 ml is the medium daily milk intake of an infant.
So far, two studies have been conducted to investigate the effects of cannabis use by lactating mothers on infant development. One did find no effects (Tennes et al. 1985), the other found slight effects on motor development, if cannabis was used more than 15 days during the first month after birth (see above, Astley and Little 1990).
The amount of THC found in breast milk of cannabis users is low despite accumaltion with regular use. Only heavy cannabis use will result in amounts that possibly are relevant for the baby. Occasional or low regular use will probably not be of relevance.